The regulated secretion of insulin by the pancreatic beta cell modulates blood glucose levels within a limited range that facilitates normal metabolism. Increased glucose concentrations are sensed upon transport across the plasma membrane by Glucose Transporter glycoprotein resulting in insulin secretion. Activation of insulin receptors on various cell types diminishes circulating glucose levels by increasing glucose uptake and utilization, and reducing gluconeogenesis in the liver. Failures within this regulatory network can result in diabetes and associated pathologic syndromes that affect a large and growing percentage of the human population.
Glucose Transporters (Gluts) are a family of transmembrane glycoproteins expressed differentially among cell types and bearing distinct affinities for glucose and other saccharide ligands. One or more Glut family members are required by all viable cells to support cellular energy requirements in quiescent and activated metabolism. Altered Glut expression or transporter function may contribute the etiology of metabolic syndromes. For example, reduced pancreatic Glut-2 expression to 10% of normal in mice using an antisense-RNA approach was reported to induce diabetes. In normal contexts, pancreatic beta cell Glut-2 is expressed predominantly at the cell surface and is not sequestered among intracellular compartments. However, decreased Glut-2 expression and intracellular accumulation have been observed in animal models of diabetes, and following administration of a high-fat diet. Glut transporter deficiency at the cell surface attenuates the beta cell response to increased glucose concentrations, resulting in deficient insulin secretion and chronic hyperglycemia that may exacerbate the development of insulin resistance. Post-translational modification of Glut expression may contribute to a mechanism underlying normal glucose homeostasis and the etiology of diabetes.
The post-translational modification of Glut-2 includes a single N-glycosylation site conserved among all homologues of vertebrate species studied. N-glycans that reach the mammalian cell surface are typically multi-branched structures produced in the Golgi apparatus by sequential saccharide linkage formation by glycosyltransferase enzymes. N-glycan branching forms the necessary scaffolding for the construction of saccharide linkage patterns that can operate as ligands for lectin receptors. With the exception of the N-glycan branch initiated by β-4 linked N-acetylglucosamine addition, other major N-glycan branch structures have been assigned specific biologic functions in humans and mice, and in some cases, roles in genetic disease (Lowe and Marth, Annu Rev Biochem. 72:643-91, 2003).
Mannosyl (α-1,3-)-glycoprotein beta-1,4-N-acetylglucosaminyltransferase (also referred to as Mgat 4 or GnT-4) is involved in N-glycan synthesis. For example, the Mgat4a-encoded GnT-4a initiates the β-4 linked N-acetylglucosamine branch on the α-3 linked core mannose (FIG. 1A). The mouse Mgat4a gene is highly conserved and selectively expressed among normal tissues and cell types, with highest levels in the pancreas and intestinal tract (FIG. 1B). The present invention is based, in part, on the discovery of dietary regulation of β-4 linked N-acetylglucosamine branch expression on Glut-2 and a role for this N-glycan structure in controlling pancreatic glucose transport and insulin secretion.